The generation of accurate voltage references is required in myriad electronic systems, such as in power supplies, data conversion circuits such as analog-to-digital converters and digital-to-analog converters, and in measurement and control systems. Voltage reference generation circuits must provide the required reference voltages having values that are, ideally, independent of power supply fluctuations and temperature changes. This is true if the electronic circuitry containing the voltage reference generation circuits is to operate properly when such power supply and temperature changes occur.
A bandgap voltage reference circuit is a popular circuit for use in generating accurate voltage references in integrated circuits. Microprocessors, memory devices, and a wide variety of other types of integrated circuits commonly found in modern electronic systems utilize bandgap voltage reference circuits. As will be appreciated by those skilled in the art, a bandgap voltage reference circuit utilizes the weighted sum of a proportional to absolute temperature (PTAT) voltage (i.e., a voltage with a positive temperature coefficient) and a complementary to absolute temperature (CTAT) voltage (i.e., a voltage with a negative temperature coefficient). A PN junction has such a CTAT voltage characteristic and thus the CTAT voltage is typically provided through the base-emitter voltage Vbe of a bipolar transistor. The PTAT voltage is typically provided through the difference in base-emitter voltages Vbe1, Vbe2 of two bipolar junction transistors Q1, Q2 operating at different current densities, namely ΔVbe=Vbe2=Vbe1. If the collector current densities of the two transistors Q1, Q2 are properly selected (i.e., collector current density of transistor Q2 greater than collector current density of transistor Q1) then the voltage ΔVbe has a positive temperature coefficient, as will be understood by those skilled in the art.
Proper operation of a bandgap voltage reference circuit assumes that the base-emitter voltage Vbe is a linear function of temperature. While this is true to a first order approximation, the voltage across a PN junction and thus in the base-emitter voltage Vbe has nonlinearities as a function as temperature, with the magnitude of these nonlinearities typically being termed “curvature.” In many applications the effect of the curvature over the required temperature range on the generated voltage reference is unacceptable and must be compensated for. “Curvature correction” is the elimination of or compensation for curvature in order to thereby eliminate or reduce the effects of curvature on the output voltage reference provided by the bandgap voltage reference circuit. Improved curvature correction methods, circuits, and systems are needed.